1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, it relates to a configuration for prevention of blocking of sheets that have been discharged onto a discharge tray without causing a reduction in image quality or productivity.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a printer, a copying machine, or the like, which executes image formation by using an electrophotographic system, transfers a toner image to a sheet, conveys the sheet to a fixing device, and fixes the toner image to thereby form an image on the sheet. Furthermore this type of imaging forming apparatus includes a two-sided (double-sided) image forming mode in which a reversing unit reverses the sheet with the image formed thereon and then a re-conveyance unit conveys the sheet again to an image forming portion to thereby execute image formation on both the front and back surfaces of the sheet.
However, in this type of conventional image forming apparatus, after fixing of the toner image, although the sheet is discharged onto a discharge tray, the sheet may not yet be sufficiently cooled. Consequently, melted toner on a sheet which is discharged onto the discharge tray may cause a blocking phenomenon with a sheet that has already been discharged onto the discharge tray. In particular, when an image is formed on both surfaces of a sheet (recording medium), toner on adjacent sheets in the discharge tray comes into direct contact and generally increases the occurrence of a blocking phenomenon. U.S. Patent Application Publication No. 2007/0196152 discusses an approach for this type of sheet blocking, for example, by providing a cooling portion that brings a sheet into contact with cooling air along the direction of sheet stacking to thereby reduce the temperature of the sheets which are discharged onto the discharge tray. Japanese Patent Application Laid-Open No. 2008-242335 discusses an apparatus which includes a temperature detection portion that detects a temperature of a sheet discharged onto the discharge tray and executes control to vary the sheet-to-sheet distance or the fixing temperature based on the temperature result detected by the temperature detection portion. In an image forming apparatus that executes this type of control, when the temperature of a sheet discharged onto the discharge tray is a temperature that will cause a blocking phenomenon, the temperature of the sheet discharged onto the discharge tray can be reduced by increasing the sheet-to-sheet distance or by reducing the fixing temperature.
However the sharp growth of color applications in recent years has created a need in the print-on-demand (POD) market, the graphics art (GA) market or the like for extremely high image quality in the images formed on sheets. Furthermore, there has been a corresponding increase in the demand for high-speed production of high-quality image sheets. When an extremely high-quality image is formed on the sheet, the amount of toner used in the image formed on the sheet is higher than the amount used in a conventional image. However, when a toner amount increases in a conventional image forming apparatus, the weight per sheet increases and a blocking phenomenon caused by blocking of adjacent sheets in a lower portion may occur due to the weight of sheets stacked in the discharge tray. When a high-quality image is formed on both sides of a sheet, since the toner on the sheets stacked in the discharge tray comes into direct contact, there is a higher possibility of a blocking phenomenon occurring. When executing high-speed production of high-quality images, if blocking of sheets is prevented, for example, by increasing the sheet-to-sheet distance, productivity will be adversely affected, and if the fixing temperature is reduced, image quality will be adversely affected.